I was asked to review code that -- well -- was evil.
Not like "shabby" or "non-pythonic". Nothing so simple as that.
We'll get to the evil in a moment. First, we have to suffer two horrible indignities.
- Busy Waiting
- Undefined Post-Conditions.
We'll beat all three issues to death separately, starting with busy waiting.
Busy Waiting
The Busy Waiting is a sleep-loop. If you're not familiar, it's this:
while something has not happened yet AND we haven't timed out:
time.sleep(2)
Which is often a dumb design. Busy waiting is polling. It's a lot of pointless doing something while waiting for something else.
There are dozens of message-passing and event-passing frameworks. Any of those is better than this.
Folks complain "Why install ZMQ when I could instead write a busy-waiting loop?"
Why indeed?
For me, the primary reason is to avoid polling at fixed intervals, and instead wait for the notification.
The asyncio module, confusing as it is, is better than polling. Because it dispatches events properly.
This is minor compared with the undefined post-conditions.
Undefined Post-Conditions
With this crap design, there are two events. There's a race between them. One will win. The other will be silently lost forever.
If "something has not happened" is false, the thing has happened. Yay. The while statement ends.
If "something has not happened" is true and the timeout occurs, then Boo. The while statement ends.
Note the there are two, unrelated post-conditions: the thing has happened OR the timeout occurred. Is it possible for both to happen? (hint: yes.)
Ideally, the timeout and the thing happening are well-separated in time.
Heh.
Otherwise, they're coincident, and it's a coin-toss as to which one will lead to completion of the while statement.
The code I was asked to review made no provision for this unhappy coincidence.
Which leads us to the pure evil.
Pure Evil
What's pure evil about this is the very clear statement that there are not enough desktop notification apps, and there's a need for another.
I asked for justification. Got a stony silence.
They might claim "It's only a little script that runs in the Terminal Window," which is garbage. There are already lots and lots of desktop apps looking for asynchronous notification of events.
Email is one of them.
Do we really need another email-like message queue?
(Hint: "My email is a lot of junk I ignore" is a personal problem, not a software product description. Consider learning how to create filters before writing yet another desktop app.)
Some enterprises use Slack for notifications.
What makes it even worse (I said it was pure evil) was a hint about the context. They were doing batch data prep for some kind of analytics/Machine Learning thing.
They were writing this as if Luigi and related Workflow managers didn't exist.
Did they not know? If they were going to invent their own, they were off to a really bad start. Really bad.